Among high energy density electrochemical cells, those containing alkali or alkaline earth metal anodes, such as lithium, have the highest energy density per pound. Such metals, however, are highly reactive with water, and must be used with organic or inorganic nonaqueous electrolyte solvents such as dioxolane, sulfur dioxide, and thionyl chloride, with the latter two also functioning as active cathode depolarizers.
The total ampere-hours delivered by a cell generally can be greatly increased by using the cell, if possible, in a secondary mode. In the secondary mode the cell is cycled through a series of discharge-recharge cycles. Since the cell is recharged after each discharge, instead of being discarded, such a cell yields many more ampere-hours than primary or nonrechargeable cells.
In order for an alkali or alkaline earth metal anode to be usefully employed in a secondary mode, the metal must replate in an acceptable form on the anode during each recharge cycle. Such metals will deposit onto an anode from an ion containing solution, such as a cell electrolyte, on the passing of a current through the solution. But in a cell, an anode metal such as lithium will ordinarily electrodeposit in the form of dendrites and form an inactive, mossy or poorly adherent powdery layer on the surface of the anode. The dendrites that are formed penetrate the cell separator and short circuit the cell. Stronger cell separators cannot alone solve this problem, since the dendrites eventually penetrate almost any cell separator.
The dendrites not only short circuit the cell, but they also separate from the anode and become inactive during recharging. It is believed that such separation occurs because of the preferential dissolution of the bases of the dendrites during recharging. The poorly adherent powdery or mossy layer is similarly less electrically active due to its poor electrical contact with the anode. Further, the mossy layer creates mass transport limitations, which also adversely affect the rechargeability of the anode.
Electrical disconnection of the dendrites and the mossy layer from the anode results in a reduction in the active mass of the anode after each discharge-recharge cycle. The discharge capacity of the cell then decreases with each discharge-recharge cycle and eventually the anode becomes totally inactive. Thus dendrite formation would remain a problem even with an ideal separator which could prevent internal cell shorting.
The rechargeability of anodes are expressed by the net efficiency of anode utilization, which can be expressed by the formula: ##EQU1## For example, in a cell containing a lithium anode and a carbon cathode current collector, the efficiency of the anode has been determined to be about 25 to 30 percent. This figure is low because of the problems recited above.
It is postulated that the formation of the dendrites and the mossy layer results from the formation of a film on the anode. Freshly deposited metal, being highly reactive, reacts with the electrolyte to form a thin insulative film on the anode. This film in turn causes an uneven current density distribution, which produces further irregular metal deposition, such as dendrites and nonadherent layers. Furthermore, the film causes preferential dissolution of the bases of the dendrites during cell discharge, thus causing the dendrites to detach themselves from the anode.
Various additives, such as the ions of metals reducible by lithium and capable of forming lithium rich intermetallics or alloys, which are disclosed in U.S. Pat. No. 3,953,302, were developed to reduce the growth of dendrites. The cycling efficiency with these additives, even though high, is still not ideal, and a cell containing these additives still has a limited number of possible discharge-recharge cycles. Furthermore, these additives may cause a reduction in cell potential of up to one and one-half volts.
Another method of reducing the dendrite problem was disclosed in U.S. Pat. No. 4,002,492. This patent discloses a cell containing a lithium-aluminum anode. The anode contains at least eight atomic percent aluminum and the cell uses selected solvents such as dioxolane to further reduce dendrite growth. The problem however, is that on recharge lithium-aluminum intermetallics are formed and such lithium-aluminum intermetallics are powdery and do not form a continuous sheet. Consequently the lithium-aluminum must be held in an inactive matrix in order to be successfully used as a rechargeable anode. The use of the inactive matrix reduces the primary energy density of the battery and increases manufacturing complexity and expense.